1. Field of the Invention
The present invention relates to a high-temperature pressure sensor, which is preferably used for detecting combustion gas pressure in a cylinder of an internal combustion engine.
2. Description of Related Art
FIG. 10(A) shows one example of a conventional high-temperature pressure sensor.
This pressure sensor comprises a housing 1a with a bore 10b surrounded by a cylindrical wall 10a, a deflection sensing element 6a provided in the bore 10b, a diaphragm 2a having a peripheral end fixed to the cylindrical wall 10a to close the opening of the bore 10b and deflectable in an axial direction of the cylindrical wall 10a when high-temperature fluid pressure acts on its surface (i.e. pressure sensing surface) A, and pressure transmitting members 7a, 8a transmitting the deflection of the pressure sensing surface A to the deflection sensing element 6a. For example, Unexamined Japanese Patent application No. HEI 4-290937/1992 discloses such a high-temperature pressure sensor.
The diaphragm 2a has the front surface A subjected to, for example, combustion gas and a reverse surface B facing the bore 10b in the housing 1a. Due to temperature difference occurring between the surfaces A and B when the diaphragm 2a receives heat energy from the combustion gas, the surface A causes a thermal expansion larger than that of the surface B. Thus, the diaphragm 2a protrudes outward (i.e. toward a combustion chamber), as shown in FIG. 10(B).
This deflection is transmitted to the deflection sensing element 6a via the transmitting members 7a, 8a, resulting in output errors. Accordingly, measuring accuracy is deteriorated.
Cancellation of the deflection of pressure sensing surface due to temperature difference between the surfaces A and B is very difficult, because the temperature difference widely changes depending on engine operating conditions.
FIG. 11 shows relationship between engine speeds and maximum surface temperatures of the diaphragm 2a according to the high-temperature pressure sensor of FIG. 10, two lines of which correspond to large and small engine load conditions, respectively. FIG. 12 shows relationship between crank angles and surface temperatures of the diaphragm 2a at a predetermined engine speed in both large and small engine load conditions. It is understood from FIGS. 11 and 12 that the temperature of the surface A of diaphragm 2a widely varies depending on engine speeds, engine loads and crank angles.
FIG. 13 shows a change of actual sensor output (solid line) and a change of actual cylinder pressure (dot line) during one complete combustion cycle. As illustrated in FIG. 13, the sensor output is smaller than the actual cylinder pressure due to adverse effect of thermal deflection of the diaphragm 2a derived from temperature difference between the surfaces A and B. Namely, combustion gas pressure generally acts as compression force on the deflection sensing element 6a while thermal deflection of the diaphragm 2a decreases this compression force unexpectedly.
Temperature of the surface A widely varies in accordance with temperature change of combustion chamber which is related to engine speeds, engine loads and crank angles. Temperature difference between the surfaces A and B also varies widely depending on engine speeds, engine loads and crank angles.
Temperature of the surface A also changes in accordance with the amount of soot accumulated on the surface A.